It is necessary on large, modern aircraft to ventilate the interior of the aircraft and to provide temperature control. Some examples of these uses include:                a) Cabin, flight deck, crew rest, lavatory, and galley ventilation and temperature control for crew and passengers        b) Flow to clear smoke from the flight deck        c) Flow to prevent lower lobe smoke from penetrating into the cabin or flight deck        d) Cargo compartment ventilation and temperature control for animals and perishables        e) Equipment cooling        
Traditionally, outside air (also known as pack air or conditioned air) has been the primary means of meeting the various aircraft air conditioning requirements. From the outside, the air is typically brought in via an engine, or an Auxiliary Power Unit (APU), bleed air extraction and one or more downstream air conditioning packs. A significant amount of energy is required to bring outside air into a pressurized aircraft during cruise where ambient pressures are low. In the case of the use of engine bleed air, the prior art systems typically elevate the air to pressures far beyond the needs of the cabin, thereby disadvantageously consuming enormous quantities of energy.
Once pressurized, this outside air is conditioned by the air conditioning packs to control the temperature of the cabin, flight deck, and in some cases the cargo bay. The larger the heat loads in these areas, the colder the temperature of, or the greater the flow rate of, the air that must be supplied. Consequently the prior art systems require (depending on the system employed) more energy, air conditioning equipment weight and (or) ram air usage drag to accommodate the larger heat loads.
For instance, one prior art system includes a cooling pack and a mixing manifold. Outside air is fed from a source of compressed outside air, such as an auxiliary power unit driven compressor or the main engine compressor, into an outside air line. The air line directs the air to the cooling pack which cools the outside air below the lowest temperature required by the air conditioning demands. Thus the air conditioning pack also removes the waste heat introduced into the air by the compression to, or above, cabin pressure. Note, also that the Bleed Air System may remove heat upstream of the air conditioning pack. From the air conditioning pack, another air line directs the compressed, cooled, outside air to a mixing manifold. In the meantime, air inside the aircraft is being warmed by heat loads and is concurrently entraining humidity.
Previously employed solutions also use a recirculation fan to collect the inside air and to feed the resulting recirculation air to a recirculation line. The recirculation line then directs the recirculation air back to the mixing manifold. Within the mixing manifold, the recirculation air and the cool, compressed, outside air mix to form inside air once again. Another air line directs the inside air flowing from the mixing manifold to the interior of the aircraft. In particular, the previously developed systems feed the inside air to all interior sections of the aircraft in parallel.
Unfortunately, as noted, the outside air surrounds the aircraft at low pressure during long distance cruise. The low pressure of the outside air imposes the burden of compressing the outside air as it is drawn into the aircraft. That compression burden adds to the need for large, heavy, energy consuming equipment onboard the aircraft where space, weight, and energy are at a premium.
Another solution entails bleeding off high pressure air from sources readily available onboard, for instance using engine bleed air. However, such sources elevate the pressure of the bleed air well above that needed for inside air. If the amount of air being compressed could be reduced, the associated energy could be more beneficially employed to propel the aircraft. In the alternative, the size or fuel consumption of the engine could be reduced.
Prior art air conditioning systems have also attempted to provide a relatively constant volumetric flow to the cabin for ventilation and cooling. Typically, the nominal, cruise, outside air, inflow rates are well above the FAR/JAR (Federal Aviation Regulations and Joint Aviation Requirements) requirements associated with cabin, flight deck, crew rest, lavatory, and galley ventilation and temperature control for crew and passengers. Moreover, the government has recently made the regulatory requirements related to maintaining the cabin and crew compartments below temperature and humidity thresholds more stringent. To meet these regulations and to address reliability concerns, backup ventilation systems have been implemented in some recent aircraft designs, thereby adding to the weight and complexity of the aircraft.
Moreover, even when the outside air is at sea level atmospheric pressure (e.g. the aircraft has landed), the outside air must still be drawn into the aircraft and cooled to provide cabin temperature control. To do so still requires running an engine or auxiliary power unit. Either of which creates noise and pollutants at airports where both noise and pollutants must be controlled. Nor is providing the outside air from ground support equipment desirable as this solution requires additional mechanical equipment in the crowded area of the terminal. In some cases the added equipment also produces noise and pollutants.
Thus, it would be desirable to reduce the need for outside air to provide onboard air conditioning.